1. Technical Field
The present invention relates to a forward converter for performing a synchronous rectification on an output current.
2. Background Art
Japanese Unexamined Patent Application Publication No. 2004-208444 discloses a conventional synchronous rectification forward converter. FIG. 1 illustrates a circuit of the converter of Japanese Unexamined Patent Application Publication No. 2004-208444.
In the synchronous rectification forward converter circuit illustrated in FIG. 1, a main switch element 1 is connected in series to a primary coil 131 of a transformer 130, and an output voltage of a secondary coil 132 of the transformer 130 which is supplied to a load 143 is controlled to be a constant voltage in accordance with an ON-OFF drive of the main switch element 1. This converter includes a switch element 150 which is connected in series with a tertiary coil 133 of the transformer 130. The converter also includes a switch element driver circuit 121 for performing an ON-OFF control for the main switch element 1 on the basis of a detection voltage of an output voltage detection circuit 119 including the switch element 150 that detects an output voltage from an induced voltage of the tertiary coil 133.
With the conventional synchronous rectification forward converter illustrated in FIG. 1, when an excess voltage is applied from the output side (backflow), an excitation of the secondary side choke coil 4 of the transformer 130 increases. An ON period of the transformer is accordingly set longer. With the transformer winding driver type circuit illustrated in FIG. 1, the ON period of the transformer is abnormally increased. Thus, self-oscillation may be caused by the choke coil 4 on the output side in response to the above-mentioned backflow. As a result, the control of the primary side switch element driver circuit 121 becomes unstable in some cases.
In addition, because of the above-mentioned increase in the ON period of the transformer, the excitation of the choke coil 4 cannot be reset in one cycle of a switching frequency in some cases. In that case, the ON period of the transformer may be extended beyond the control range of the switch element driver circuit 121, so that the excitation state of the transformer 130 cannot be reset. Thus, an excess drain voltage may be generated for the main switch element 1. This may cause a stress on the main switch element 1, which leads to additional problems.